It isn't as relevant in baseball, because within a division, teams play more or less the same schedules. Teams in the Pirates' division, for example, play 19 games against each of their Central divisional foes (that's 74 games), 34 games against each of the Eastern and Western divisions (another 68), and 20 interleague games against the American League Central. The schedules outside their home divisions aren't exactly balanced, but they're fairly close. So every team plays about the same schedule, and if everybody plays the same schedule, strength of schedule isn't all that relevant.
When it does become important, though, is during the season. Teams don't play the same schedule all season long. Some teams play easier foes in the first half of the season, some player easier foes in the second half, and some play a fairly balanced schedule. How about in 2015?
To answer this, I looked at the remaining games (after today) for each team in the National League Central. For example, here's the remaining schedule for the Pirates and each foe's winning percentage entering play today:
- Ten against the Reds (.455)
- Nine each against the Cardinals (.637) and Cubs (.539)
- Seven each against the Brewers (.435) and Rockies (.435)
- Six against the Dodgers (.565)
- Four each against the Giants (.527), Marlins (.418), and Nationals (.551)
- Three each against the Diamondbacks (.472), Mets (.516), and Royals (.607)
- Two against the Twins (.549)
If I multiply each of those winning percentages by the games against the team and divide the sum by the games remaining for the Pirates after today, 71, I get a weighted average won-lost record of .514 for the teams the Pirates will face the rest of the season.
How good is that? It's pretty good. The National League, the teams with the best won-lost records for the rest of the season are the Reds (.539), Rockies (.518), and Pirates (.514). (The poor Reds have 13 games against the Cardinals, ten each against the Cubs and Pirates, seven against the Dodgers--all playoff contenders--as well as four games against the Mets and three against the Giants, both of which have winning records.) The easiest schedules are those of the Mets (.469), Padres (.473), and National (.475). (The National League East has the two worst teams in the league so far, the Marlins and Phillies, against whom the Mets and Nationals still have 19 games each while the Padres have 13, helping explain their relatively easy schedules.)
Here's a way of looking at this: The Pirates' foes have a winning percentage of .514. Multiply that by their 71 games remaining, and the Pirates are playing teams that could be expected to win .514 x 71 = 36.5 games and lose 34.5. Let's do this exercise for the entire National League Central:
Team W-L W L GB
Chicago .488 35.1 36.9 ---
St. Louis .490 34.3 35.7 0.2
Milwaukee .513 35.4 33.6 1.8
Pittsburgh .514 36.5 34.5 1.9
Cincinnati .539 38.8 33.1 3.7
The games behind column is the opposite of what you'd normally see, as it displays the number of games each teams trails the Cubs, who have the easiest remaining schedule in the division. The Reds' remaining schedule is against teams that are 3.7 games better than the Cubs' foes applied to the remainder of the schedule. The Pirates trail the Cubs by 1.9 games and the Cardinals by 1.7 games, indicating that the Bucs' remaining schedule is about two games tougher than that of their divisional opponents. That'll make the task of catching the Cardinals (they trail by 4.5 games entering play today), and holding off the Cubs (they're 4.5 games ahead) that much harder.